1. Field of the Invention
This invention relates to a new process for the production of oligourethanes containing mercapto groups from selected starting materials and to the use of the products obtained by this process as binders for coating and sealing compositions hardenable by oxidation and as additives for epoxide resins.
2. Description of the Prior Art
Oligomers and polymers which contain SH-groups and which may therefore be hardened by oxidation with oxidizing agents, such as MnO.sub.2, PbO.sub.2, chromates or organic peroxides, or even by air in the presence of certain catalysts to form rubber like elastic plastics have already been repeatedly described. The use of binders of this type for the production of sealing and coating compositions is also known (cf German Offenlegungsschrift Nos. 2,739,117 and 2,557,088 and German Auslegeschrift No. 2,363,856).
The main advantage of binders such as these over other types of binder (such as for example oligourethanes containing terminal NCO groups) lies inter alia in the fact that they harden and form bubble-free polymers even in the presence of water which may reach them through moist pigments or additives or from the atmosphere.
On the other hand, oligourethanes containing free NCO groups which are used as binders for sealing and coating compositions have several advantages to compensate for the disadvantage of bubble formation where hardening is carried out with water (i.e. atmospheric moisture). They are suitable for example for the production of highly weather-resistant compositions characterized by high permanent elasticity, extreme toughness, high abrasion resistance and resistance to solvents and other chemical influences.
Accordingly, there has been no shortage of attempts to link the valuable properties of the polyurethanes with the hardening mechanism through mercapto groups. Oligourethanes containing terminal mercapto groups are described in particular in German Offenlegungsschrift No. 1,770,259. They are produced by initially reacting a polyol with excess diisocyanate to form a prepolymer containing NCO-groups and then urethanizing the prepolymer thus formed with an alcohol containing an HS-group. The process according to German Offenlegungsschrift No. 1,770,259 gives oligourethanes containing terminal mercaptan groups which harden through oxidation without any bubbles and without foaming, even in the presence of moisture.
However, the products obtained by the process according to German Offenlegungsschrift No. 1,770,259 have disadvantages which make them difficult or impossible to use for high quality sealing or coating compositions. Above all, the products are highly susceptible to yellowing because aromatic diisocyanates are predominantly used in their production. The known yellowing of these isocyanates has a very unfavorable, quality-reducing effect in the case of coating compositions containing white or light pigments. Although aliphatic diisocyanates are also mentioned as suitable and an aliphatic diisocyanate is used in one Example, these starting products (as will also be shown hereinafter in comparative tests) do not in any way lead to optimally suitable sealing and coating compositions. One of the major disadvantages of using the aliphatic diisocyanates accordinq to German Offenlegungsschrift No. 1 770 259 is the high surface tackiness of the fully reacted compositions. Precisely the same statement can be made in view of the teaching of U.S. Pat. No. 3 446 780. The authors of this reference as well as the inventors of above Offenlegungsschrift did not find a suitable way to sealing and coating compositions which are lightfast and whioh lead to fully reacted compositions which are free from surface tackiness. In addition, the authors of German Offenlegungsschrift No. 1,770,259 failed to recognize that mercapto ethanol in particular has significant advantages as a reactant for the isocyanate prepolymer and, under certain conditions, leads to particularly suitable products. Instead, it is alleged that the use of mercapto ethanol leads to substances which have only a minimal resistance to alkalis and heat (German Offenlegungsschrift No. 1,770,259, page 16).
It has now surprisingly been found that, by using selected starting materials, particularly 1-isocyanato- methyl-1,3,3-trimethyl-5-isocyanatocyclohexane (isophorone diisocyanate, IPDI) as the diisocyanate in conjunction with certain polyether polyols described in detail hereinafter and particularly in conjunction with 2-mercapto ethanol, it is possible to produce particularly valuable oligourethanes containing mercapto groups which may be used in particular in the production of light-stable coating and sealing compositions characterized by excellent stability to alkalis and heat.